German patent 3,424,825 describes a process whereby the laser intensity is maintained between defined limit values. For example, the intensity of the plasma luminance is detected continuously by a radiation sensor, and the laser intensity is regulated by modulation of the laser radiation in order to maintain the plasma and prevent an unwanted detonation wave. The control principle of this process is based on a two-point controller with the known disadvantages such as the dependence of the cutoff frequency of the controller on the time constant of the control circuit. Therefore, this process is not suitable for precision adjustment of the laser intensity in order to achieve precision removal of material from a workpiece.
German patent 3,926,859 discloses a process and a device for cutting or perforating workpieces, especially those made of metal, with a laser beam, where the machining point on the workpiece is monitored with a radiation sensor that measures the prevailing workpiece temperature by detecting the heat radiation. This patent describes a two-point controller that cuts off the laser beam on reaching an upper limit for the workpiece temperature and turns the laser on again on reaching a lower workpiece temperature. Therefore, the laser is pulsed by reaching the upper and lower workpiece temperatures.
It is known that lasers need a certain amount of time from stimulation to emission=of the laser radiation. This is especially true of solid-state lasers (e.g., an Nd:YAG laser) and for gas lasers (e.g., a CO.sub.2 laser). With solid-state lasers the delay time or lag is higher by approximately one power of ten. In addition, the laser continues to pump after the laser beam has been switched off. The cutoff frequency of the two-point controller is determined by these specific time constants of the laser which also change with the pulse frequency of the laser (according to German patent 3,926,859). For this reason, the cutoff frequency of the laser pulses cannot be raised above a certain level because otherwise the laser pulses would overlap and would thus raise the minimum energy level in the machining operation. Due to the laser-specific time constants when using a two-point controller for controlling the laser beam, it is necessary to operate with a low cutoff frequency, which leads to relatively large power pulses. Therefore, the amount of material removed from the workpiece in the forward movement of the laser over the material in one layer cannot drop below a certain limit.
When the laser beam strikes the material, there is an increase in temperature within a very short period of time which is in the range of nanoseconds. The control parameter for controlling a laser such as the beam power must respond at least equally rapidly. If this is not the case, then such a control system becomes unstable.
The dynamics of the laser, that is, the system-specific response time and the continued pumping of the laser radiation after the system has been shut down, are effective in the control circuit with a two-point controller according to German patent 3,926,859, which detects the radiation of one pulse and regulates the same pulse with the signal thus derived, but this leads to laser pulses that cannot be reproduced accurately. Therefore, a controller that contains the time constants of the laser in the control circuit is not suitable for precision removal of material by machining in this way.
In addition, the inherent dynamics of lasers are approximately constant only at pulse frequencies that vary within certain very narrow limits. If a self-pulsing laser is used, then the frequency of the laser pulses changes constantly. Thus, the response time of the laser to stimulation pulses also varies, which thus makes precision machining with a very small depth of removal of material such as 1 .mu.m virtually impossible.